A U.N. conference aimed at improving the Nuclear Non-Proliferation Treaty has been stalled again by Iranian objections to the agenda.

Conference chairman Yukiya Amano says the meetings in Vienna have been postponed until Friday. At issue is Iran's refusal to accept a phrase calling for the "need for full compliance" with the treaty.

Diplomats say Iran is afraid of being singled out by the phrase.

Amano says he will continue to hold informal talks with delegates. This is the second postponement since the conference opened on Monday.

The U.N. Security Council has imposed sanctions on Iran for its refusal to halt uranium enrichment, a process that can be used to make material for nuclear weapons. Tehran says its atomic program is for civilian nuclear energy, as permitted by the Nuclear Non-Proliferation Treaty.

The two-week conference in Vienna is intended to prepare for a general review of the Nuclear Non-Proliferation Treaty in 2010.

An earlier attempt to overhaul the treaty collapsed in 2005 due to a similar procedural impasse.

Decisions on the agenda of meetings must be reached by consensus. Western powers and members of the Nonaligned Movement, of which Iran is a member, have agreed to the agenda. But talks can not go forward without Iran's approval.

The Nuclear Non-Proliferation Treaty was signed in 1970 to stop the spread of nuclear arms. Members who did not already possess nuclear weapons pledged not to pursue them, while the five original nuclear powers (U.S., Russia, Britain, France and China) agreed to phase out their arsenals.